1. Field of the Invention
This invention relates in general to optical sensing arrays for detecting the movement or intrusion of objects into guarded zones. More particularly the invention relates to optical sensing arrays such as light curtains of the type that provide protection for human operators who are working with machines and other industrial equipment.
2. Description of the Related Art
Optical sensing arrays such as light curtains employ infrared beams to provide operator safety in a variety of industrial applications. The optical sensing arrays typically are employed for operator protection around machinery such as punch presses, brakes, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. The systems employ light transmitters having light emitting diodes that are mounted at spaced positions along one side of the guard zone together with light receivers mounted at the opposite side of the zone. Modulated infrared light beams are strobed along separate parallel channels toward the light receivers. When the beams are blocked by penetration of an opaque object, such as the operator's arm, the control system shuts the machinery down, prevents the machine from cycling or otherwise safeguards the operator from injury, or the machine and/or work piece from damage.
In light curtains and other similar optical sensing arrays it is desirable to control the maximum angle of acceptance, which is the maximum angle between the transmitted light beam and the light receiver element at which the system can be expected to properly operate. Relatively large angles of acceptance are undesirable in light curtains because they make it more difficult for the control system to properly discriminate between signals from the light beams and signals from spurious or transient sources such as light reflections from the work piece or surrounding environment. Such conditions could cause the control system to detect the spurious light and produce a false signal that the light beams are unbroken when in fact objects are penetrating the guarded zone. This is a highly unsafe condition in that the light curtain system would then not properly shut down or safeguard the area when the person's arm or other object penetrates through the light beams.
Safety laws and regulations in many states and countries are in effect or have been proposed requiring that light curtain systems, before they can be certified for sale and use, meet certain minimum criteria for angle of acceptance accuracy. Typically the maximum allowed acceptance angles have been on the order of 4.degree. full angle. Many countries in Europe now require that the angular accuracy for light curtains be no more than 21/2.degree., and Australia requires that the angular accuracy be no more than 2.degree..
In prior art light curtains the light beams are detected by light receivers, typically phototransistors (PT's) in a circuit which converts amplitude of the incident light beam into a voltage signal. With the light beam properly aligned on the optical axis of the PT, the detected signal is generally at its strongest. Misalignment of the light beam from that optical axis, such as from displacement of either the light transmitter or light receiver, causes a drop off in signal strength as a function of the misalignment angle. Where (he light transmitter and receiver bars are maintained apart at a known distance then the displacement angle of the light beam from the PT's optical axis can be roughly measured as a function of signal strength. However, the absolute signal strength also varies as a function of the Square of the distance range between the transmitter and receiver. For example, when the range varies from one foot to 300 feet then the signal would vary over the range from about 100,000 to 1. Electronic control circuits relying upon absolute signal strength would not be able to discriminate between transmitter and receiver bars which are far apart and properly pointed, and those that are very close together and mispointed because the signal strength in both cases could be the same. This can lead to conditions in which the light curtain does not properly safeguard the area. For example, a light curtain in which the transmitter and receiver bars are mispointed beyond the maximum 4.degree. limit but at close range could only have a signal attenuation of 1,000 to 1. Absolute signal strength is thus not a good measure of light beam angular accuracy.
In the prior art, one conventional arrangement for controlling angular accuracy of light beams is through the use of high quality optics utilizing collimating lenses to focus the beam with the LED in the focal plane of the lens system. However, it is difficult to maintain sharp edges on the beams because of built-in features associated with the LED's, such as bond wires and epoxy, variations in emissions across the face of the LED surface, dirt on the LED surface, focusing problems and the like. The problem is exacerbated when a large number, which can be on the order of one hundred twenty, of the LED's are mounted on a single transmitter bar. The result is that small angles of mispointing of the beam to the PT's optical axis gives relatively small signal attenuation. For example, at a mispointing angle of 21/2.degree. the attenuation is on the order of 30%. This is very small in relation to the 100,000 to 1 attenuation that can be expected where the distance between the transmitter and receiver bars can range from one to 300 feet. Therefore a circuit which uses the attenuation edge of the signal amplitude to determine beam angle is an unreliable method for controlling angular accuracy in light curtains.
The prior art systems employing high quality optics with large and precise lenses do not provide a satisfactory solution to the foregoing problem for a number of reasons. Any minor defects in the lenses produce softness on the edges of the light beams, which limits the use of signal attenuation for determining angular accuracy. The problem is exacerbated as a result of the trend in the industry to produce smaller light curtain products in which the transmitter and receiver elements are mounted close together such that the larger collimating lenses cannot be employed. In certain of these small size light curtains the dies of the PT's and LED's are mounted directly on the printed circuit (PC) boards of the light transmitter and receiver bars. In these arrangements any misalignment of the transmitter or receiver bars moves all of the respective PT's or LED's so that all would be misaligned, thereby compounding the problem of controlling angular accuracy.
The need has therefore been recognized for an optical sensing array and method of operation which can accurately control the acceptance angle of light beams and in which the control of angular accuracy can be maintained over a wide range of distance between the light transmitter and receiver elements. Despite the various types of optical sensing arrays in the prior art there has not yet been provided a suitable and attractive solution to these problems.